1. Technical Field
The present invention relates to an electromagnetic field superimposed lens for an electron beam device capable of high resolution observation, and to an electron beam device using this electromagnetic field superimposed lens.
2. Related Art
In order to perform inspection or observation of the shape of a fine pattern using an electron beam device, various electron beam devices such as a scanning electron microscope have conventionally been used, and in particular, there has been a high demand for high resolution observation accompanying the fact that electron beam devices have become ultra fine in recent years. As an electromagnetic lens enabling high resolution observation, Japanese patent laid open No. Hei. 6-24106 disclosed a structure where a decelerating electrical field is caused to overlap a lens magnetic field, reducing the spherical aberration factor Cs and chromatic aberration factor Cc.
As shown in FIG. 5, this electromagnetic field superimposed lens is formed from a bi-potential lens (electrostatic immersion lens) comprising a magnetic field lens ML, an electrode RE, and a lower magnetic pole UP, and has a structure where a magnetic field is generated in a space containing a pole piece gap PS if electric current flows in a coil SP. A cylindrical electrode RE is provided coaxially with an optical axis OA inside an upper pole piece OP via an insulator IS. A decelerating electric field is formed across the electrode RE by applying a positive potential to the electrode RE and keeping a lower pole piece UP at ground potential. A lens having small spherical aberration factor Cs and chromatic aberration factor Cc is obtained from the action of superimposing a magnetic field occurring across the pole piece gap PS and the decelerating electric field.
However, with the above described electromagnetic field superimposed lens, there is a problem in that the device structure becomes complicated and it is easy for damage to occur. Specifically, since a sample and a lens electrode facing the sample are held at ground potential, in causing superimposing of a decelerating electrical field having the effect of decreasing aberration factor on a magnetic field, a positive high potential must be formed inside the lens barrel, but with the structure shown in FIG. 5 a positive high potential +9 kV is applied to the electrode RE and an electrode gun anode.
Although not shown in FIG. 5, in an actual lens barrel a vacuum tube, a collimating lens, a deflector, an air lock valve and a movable aperture are arranged between an electron beam generator and an objective lens. Accordingly, when a high voltage is applied to each of these elements, as described above, measures, such as providing an optical system, are required to cope with this. Further, problems arise such as damage and electron beam charging due to electrical discharge from the high voltage sections, and it is easy for disadvantages such as cost increase, and increase in the frequency of maintenance to occur to a significant increase in the number of components. There is proposed a structure where midway along the inside of lens barrel, the potential falls from a high potential to ground potential with advancement of an electron beam and a high potential is applied again to the electrode RE, but it becomes necessary to take into consideration a lens operation at voltage varying sections, and there is a separate problem that an electron optical system is made complicated.